Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
deflection ratio, Cx; the blowdown, exhaust and transfer port time areas of those ports (vide
Sec. 6.1); the deflector side clearance, rCy-rd, and the deflector radius, rj; the clearance volume
(cm 3 ) and the chamber height above the piston at tdc; the heights, hj, hx and hs, of the
deflector, the exhaust ports and transfer ports, respectively; the areas, Axp and Asp, of the
exhaust ports and transfer ports, respectively.
A change of data is invited by the programmer for any or all of the input data, as is their
recomputation. The code for the data change is the name of that input data parameter. A
printout of the entire screen display is available, which is drawn to scale from the input and
output data. The design process is aided by the computer program immediately drawing the
sketch of the piston and the porting to scale on the computer screen as a function of the input
data.
As a supplement to the general discussion on unconventional cross scavenging in Sec.
3.5.3, in Fig. 3.34(a) the breadth of the deflector at 24 mm for a bore of 48 mm means that the
piston can be cast with some 6 or 7 mm thick walls all over the crown without encouraging the
thick, solid deflector which accompanies the design of the piston in conventional cross scavenging.
The availability of a more compact combustion chamber is also evident, albeit with
some complexity in the design of the cylinder head spigoted into the cylinder bore. The lack
of the scavenge leakage path to the exhaust port, of dimension xc, is also to be observed. Just
as important is the fact that this design will scavenge well at high bore-stroke ratios as the
scavenge characteristic called GPBDEF in Figs. 3.12 and 3.13 is for an engine with a borestroke
ratio greater than 1.3.
3.5.4 QUB-type cross scavenging
A preliminary discussion of this type of scavenging was made in Sec. 1.2.2 and shown in
Fig. 1.4. It is clear that the radial disposition of the transfer ports will raise the port width
ratio, Cpb, and the value for such designs is usually about 1.0. While one can use the straightin
port arrangement, it has been demonstrated that the scavenging of such a design is actually
worse than conventional cross scavenging [1.10]. There are further, unpublished, engine tests
from QUB which would substantiate that statement. The radial arrangement of scavenge ports
has been shown [1.10] to have excellent scavenging characteristics by comparison with good
quality loop scavenging and uniflow scavenging, as demonstrated in Figs. 3.12 and 3.13.
The criterion used by the designer to optimize the scavenging process, prior to conducting
a confirmatory test on a single-cycle test apparatus, is very similar to that followed for
conventional cross scavenging. The deflection ratio, CA, calculated from Eq. 3.5.3 is still
pertinent, and the determination of it, for this more complex geometry, is aided by computer
program, Prog.3.3(b).
3.5.4.1 The use ofProg.3.3(b), QUB CROSS PORTS
The design process is best illustrated by the example shown in Fig. 3.34(b) for a 70 mm
bore and stroke "square" engine. The figure shows the screen display from the program,
slightly accentuated to show the deflector flow area by shading that portion of the piston
crown. By requiring input values for cylinder bore, stroke, con-rod length, transfer port timing
and transfer port corner radii, the program accurately calculates the total scavenge port
height and area, hs and Axp, as 15.1 mm and 1062.8 mm 2 , respectively. The deflector width
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